1. Field of the Invention
The present invention generally relates to printing apparatus, and, more particularly, to an improved screen printing apparatus, its method of manufacture, and its method of use.
2. Description of the Prior Art
Stencil screens are extensively employed to apply coatings to surfaces of mass-produced articles of manufacture. The coatings may be applied so as to produce simple schemes or patterns or they may be applied in more elaborate decorative designs including manufacturer's names, product names, logos, and similar types of information. During printing, a printing or coating medium such as ink or a thermally responsive material is forced by means of a squeegee through exposed areas of the screen which delineate the intended design to be transferred onto the surface of the article.
Examples of prior art stencil screen printing apparatus are shown in U.S. Pat. No. 3,894,487, U.S. Pat. No. 4,137,842 and U.S. Pat. No. 4,520,727. Each of these patents disclose apparatus having a metallic wire mesh design-bearing stencil screen which is secured to a metallic frame. In operation of these apparatus, the stencil screen is pushed slightly outwardly and drawn taut by the squeegee during each design transfer application. Upon completion of the transfer application, the squeegee is retracted and the screen is permitted to relax to its original condition. During their useful service lives, such screens are adaptable to screen printing a wide variety of surfaces and/or articles of manufacture. Unfortunately, particularly in printing flat substrates, the continuous tensioning and relaxing of the wire mesh screen material soon causes unavoidable and harmful fatigue of the material (particularly where the material is in contact with the lower edges of the metallic frame), hence leading to premature failure of the screen.
A further disadvantage of stencil screen apparatus of this type is that the wire mesh screen cannot always be tensioned uniformly over the entire area of the frame to which it is secured. For example, the screen is usually tensioned and cemented along substantial portions thereof to a rubber boot which surrounds the frame while the remaining portions of the screen are held in metal compression clamps which act as bus bars at times when the screen is heated by electrical resistance, such clamps usually being adjustably secured by screws to opposite ends of the frame. Due to its inherent non-suppleness, metallic wire mesh screen is difficult to attach to the rubber boot under sufficient tension to cause the screen to tightly conform to the shape of the frame. Consequently, the wire mesh screen can only be effectively tensioned along its length dimension where its clamped ends are adjustably connected to the frame by the aforementioned screws. Such non-uniform tensioning of the screen has been known to produce wrinkles and/or other variably-tensioned areas in the screen which deleteriously affect the quality of the image which is transferred by the screen to the article.
Such screens further have relatively limited inherent resiliency. This generally does not create a problem during many printing operations. However, at times when thermally responsive printing medium is applied by the screen printing apparatus, i.e., at times when the screen and printing medium are heated, the thermally responsive printing medium, if applied to a substantially cooler surface such as glass, for example, quickly sets upon contact therewith and causes the screen to adhere to the surface of the article. A practical example of where this situation physically occurs is in manufacturing plants wherein ultraviolet or colored strips of coating are applied to windshield glass, and the like. Under such conditions, retraction of the screen from the article surface usually causes removal of at least a portion of the printed image from the article surface thus essentially destroying the printed image.
In such thermally dependent printing operations, it would be most advantageous if the screen structure could be provided with greater inherent resiliency whereby it would positively and virtually instantaneously return to its initial position upon passage of the squeegee to thereby prevent sticking or blocking of the screen to the cooler article surface.
U.S. Pat. No. 4,373,441 and German Offenlegungsschrift No. 29 16 391 disclose composite stencil screen apparatus which require elaborate constructions involving electroplating opposite end portions of a metallic stencil screen that are thermoplastically secured to a thermoplastic synthetic resin material such that the thermoplastic securement between the wire stencil screen the resin material is not damaged during electrical resistance heating of the stencil screen.
An advantage exists, therefore, for a stencil screen printing screen construction which is: 1) resistant to metal fatigue, 2) uniformly tensioned across its length and width dimensions, and 3) high in inherent resiliency.
It is accordingly an object of the present invention to provide a stencil screen printing apparatus having a printing screen construction which is resistant to metal fatigue and, therefore, of long service life.
It is a further object of the present invention to provide a stencil screen printing apparatus having a printing screen construction which is uniformly tensioned in all directions within the plane of the stencil screen to thereby provide a high-quality image transfer surface.
It is a further object of the present invention to provide a stencil screen printing apparatus having a printing screen construction which is high in inherent resiliency and, therefore, especially useful in thermally dependent printing applications wherein a substantial temperature differential exists between the screen and the article surface being coated; such a highly resilient printing screen construction thereby preventing adhering of the screen to the article surface which is caused by the rapid cooling and setting of the thermally responsive printing medium upon contract with the cooler article surface.
It is another object of the present invention to provide a stencil screen printing apparatus having a printing screen construction which permits accurately controllable and essentially uniform electrical resistance heating of the screen in the instances where the printing medium is thermally responsive.
It is yet a further object of the present invention to provide methods of manufacture of a stencil screen printing apparatus which will produce an apparatus capable of fully attaining all of the aforenoted objects.
It is still a further object of the present invention to provide a method of coating article surfaces with at least one color of thermally responsive printing medium at speeds heretofore unattainable by the printing screen apparatus of the prior art through the use of at least one stencil screen printing apparatus having the novel structural features of the present invention in order to achieve rapid and effective printing of articles, such method finding particular advantage in high-volume production environments.
Still other objects and advantages will become apparent in light of the attached drawings and the written description of the invention presented herebelow.